In this kind of technical filed, research and development of a next-generation mobile communication system of the 3rd-generation mobile communication system have been conducted by 3GPP (3rd Generation Partnership Project) which is a standards body of the Wideband Code Division Multiple Access (W-CDMA) System. Especially, as the next-generation communication system of the W-CDMA (Wideband Code Division Multiple Access) system, the HSDPA (High Speed Downlink Packet Access) system and the like, research and development of the LTE (Long Term Evolution) system have been conducted at high speed. In the LTE system as a radio access system, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme are used in the downlink communications and the uplink communications, respectively (see, for example, Non-Patent Document 1).
The OFDM scheme is a multi-carrier transmission scheme in which a frequency band is divided into plural sub-carriers having narrower frequency bands, and data are mapped onto the sub-carriers so as to be transmitted. By arranging the sub-carriers on the frequency axis in a manner such that the sub-carriers are orthogonal to each other, a faster transmission rate can be achieved and frequency use efficiency can be improved.
The SC-FDMA scheme is a single-carrier transmission scheme in which a frequency band is divided into plural narrower frequency bands so that the divided frequency bands are allocated to plural user equipment (UE) terminals. As a result, the user equipment (UE) terminals can transmit using different frequency bands from each other, thereby reducing the interference between the user equipment (UE) terminals. Further, in the SC-FDMA scheme, a range of the fluctuation of the transmission power may be made smaller; therefore, lower energy consumption of terminals may be achieved and a wider coverage area may also be obtained.
In uplink communications, signals in a cell are orthogonally transmitted based on the SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme. However, in other cells, the same frequency band may be used. Therefore, it may be necessary to adequately control the other-cell interference. To that end, it may be desirable to carefully control the transmission power level of the user equipment (UE) terminal especially located at the edge of the cells.
In a general mobile communication system, from the viewpoints of promoting the increase of link capacity and battery saving opportunities in the user equipment (UE) terminals, transmission power control (TPC) is being performed. As the transmission power control (TPC), there may be two types of control methods: one is open-loop control in which the control is performed with a relatively long cycle period, and the other is closed-loop control in which the control is performed with a relatively short cycle period. The open-loop control may be preferably conducted so as to reduce the influence not depending on instantaneous fading such as distance attenuation and shadowing. On the other hand, the closed-loop control may be preferably conducted from the viewpoint of the requirements of a quick response to the influence of fading and a setting error of the transmission power level of a user equipment (UE) terminal. To improve the accuracy of the transmission power control (TPC), it may be preferable to use these two control methods together.
In so-called a circuit-switching communication system such as the W-CDMA system, a specific dedicated channel is allocated to a user equipment (UE) terminal, and the transmission power level of the user equipment (UE) terminal is gradually adjusted based on a past continuous record of the user equipment (UE) terminal. On the other hand, in so-called a packet exchange communication system such as the LTE system, no specific dedicated channel is allocated to a user equipment (UE) terminal. Therefore, in the transmission power control (TPC) for the uplink of the LTE system, the user equipment (UE) terminal transmits a sounding reference signal (SRS) to a base station apparatus across the entire system bandwidth at short intervals (e.g., every 2 ms). The base station apparatus measures the received quality of the Sounding Reference Signal (SRS), and determines the degree to which the transmission power level is to be changed from a reference value when the user equipment (UE) terminal transmits a Physical Uplink Shared Channel (PUSCH) next time. The reference value is the transmission power level determined in the open-loop control.(Transmission power level)=(reference value)+(correction value)
This correction value (offset) is expressed in a TPC bit pattern. The TPC bit pattern is transmitted to the user equipment (UE) terminal in a Physical Downlink Control Channel (PDCCH) (L1/L2 control channel). Otherwise, the TPC bit pattern may be transmitted as data of an uplink scheduling grant in the Physical Downlink Control Channel (PDCCH). In the W-CDMA system, one bit is allocated to the TPC bit pattern, and the transmission power level is corrected (changed) by, for example, one dB at a time. However, in the LTE system, transmission intervals of the user equipment (UE) terminal, (i.e., correction intervals of the transmission power level) are discretely distributed. Because of this feature, the range of the correction values may be wide, and therefore, a greater number of bits may be required to be allocated. As a result, the influence of the TPC bit pattern on a control traffic amount may become large. Therefore, it is desirable that the TPC bit pattern be transmitted without wasting bits.
On the other hand, in uplink of the LTE system, a Synchronous Hybrid Automatic Repeat reQuest (HARQ) method is also conducted. In this retransmission method, the timing to transmit the retransmission packet is determined in advance. For example, a packet may be retransmitted as a retransmission packet in a frame six frames later than the frame in which the packet has been initially transmitted as an initial packet. In this case, as described above, the uplink transmission control is performed on the initial packet so that the initial packet can be transmitted with an appropriate transmission power level. This transmission power control (TPC) based on the above-described method may also be applied to the retransmission packet.
FIG. 1 schematically shows where a user equipment (UE) terminal receives the uplink scheduling grant and then, the user equipment (UE) terminal transmits the Physical Uplink Shared Channel (PUSCH). More specifically, first, the user equipment (UE) terminal receives the uplink scheduling grant (UL-grant1). Then, based on the scheduling information in the UL-grant1, the user equipment (UE) terminal transmits an initial packet (i.e., PUSCH). The transmission power level of this transmission is determined based on the TPC bit pattern (having x bits) in the UL-grant1. However, there may be a case where the initial packet (PUSCH) has not been normally received by the base station apparatus and, as a result, the NACK signal is transmitted to the user equipment (UE) terminal via the PDCCH. In this case as well, the user equipment (UE) terminal transmits the retransmission packet based on the scheduling information in the uplink scheduling grant (UL-grant2). The transmission power level of this transmission is determined based on the TPC bit pattern (having the same x bits) in the UL-grant2.
However, there are many bits in the TPC bit pattern, and there is little time difference between the transmission of the initial packet and that of the retransmission packet. When considering those matters, from the viewpoint of effectively performing the power control while minimizing the number of bits of the TPC bit pattern, it may not be preferable that the transmission power levels of all the retransmission packets are regulated under the same transmission power control as that for the initial packet. Therefore, the TPC bit pattern may be omitted for the retransmission packets.
On the other hand, in the LTE system, it is not always the case that the frequency band (i.e., resource blocks) used for the initial packet is the same as that used for the retransmission packet. When viewed otherwise, when a resource block not the same as that used for the initial packet is used for the retransmission packet, a more enhanced frequency diversity effect may be obtained, and therefore, reliability may be improved. However, when different resources are used for the initial packet and the retransmission packets, the appropriate transmission power level of the initial packet is not always the same as that of the retransmission packets; in other words, it is normal that the appropriate transmission power level of the initial packet may differ from that of the retransmission packets. If this is the case, all of the initial packet and the following retransmission packets may be required to be separately controlled. However, this separate control may work against the demand for minimizing the number of bits of the TPC bit pattern.
Non-Patent Document 1: 3GPP TR 25.814 (V7.1.0), “Physical Layer Aspects for Evolved UTRA,” September 2006.